A Threshold Shear Force for Calcium Influx in an Astrocyte Model of Traumatic Brain Injury

Abstract

Traumatic brain injury (TBI) refers to brain damage resulting from external mechanical force, such as a blast or crash. Our current understanding of TBI is derived mainly from in vivo studies that show measurable biological effects on neurons sampled after TBI. Little is known about the early responses of brain cells during stimuli and which features of the stimulus are most critical to cell injury. We generated defined shear stress in a microfluidic chamber using a fast pressure servo and examined the intracellular Ca²⁺ levels in cultured adult astrocytes. Shear stress increased intracellular Ca²⁺ depending on the magnitude, duration, and rise time of the stimulus. Square pulses with a fast rise time (∼2 ms) caused transient increases in intracellular Ca²⁺, but when the rise time was extended to 20 ms, the response was much less. The threshold for a response is a matrix of multiple parameters. Cells can integrate the effect of shear force from repeated challenges: A pulse train of 10 narrow pulses (11.5 dyn/cm² and 10 ms wide) resulted in a 4-fold increase in Ca²⁺ relative to a single pulse of the same amplitude 100 ms wide. The Ca²⁺ increase was eliminated in Ca²⁺-free media, but was observed after depleting the intracellular Ca²⁺ stores with thapsigargin suggesting the need for a Ca²⁺ influx. The Ca²⁺ influx was inhibited by extracellular Gd³⁺, a nonspecific inhibitor of mechanosensitive ion channels, but it was not affected by the more specific inhibitor, GsMTx4. The voltage-gated channel blockers, nifedipine, diltiazem, and verapamil, were also ineffective. The data show that the mechanically induced Ca²⁺ influx commonly associated with neuron models for TBI is also present in astrocytes, and there is a viscoelastic/plastic coupling of shear stress to the Ca²⁺ influx. The site of Ca²⁺ influx has yet to be determined.